The use of turbines to generate power is economical in many situations where more conventional powerplants cannot be contemplated. To their great credit, they generally operate with minimal generation of NOx and can utilize fuels, such as digester and landfill gases. However, they do generate regulatory significant amounts of NOx, and efforts are made to reduce emissions to even lower levels.
Digester and landfill gases are gaseous by-products, principally comprised of methane and carbon dioxide, of anaerobic decomposition of organic materials from sewage treatment or landfills. These are not clean fuels according to criteria that utilities normally consider. Trace quantities of offensive compounds are typically found in the gases and often include hydrogen sulfide, ammonia and acid gas forming compounds. In addition, some compounds present in the gas and are known to clog NOx reduction catalysts and shorten the life of the turbines. The net effect for operators is that utilization of this low-cost fuel can present additional costs in terms of shortened turbine life, corroded ductwork and fouled catalysts.
Unless these costs can be recovered by taking good advantage of the energy value of these fuels, the environment will suffer and their energy value will likely be replaced with imported petroleum. It is, therefore, advantageous that all costs be recovered by the facility operators, burned in combustion devices such as turbines to either generate electricity or directly power equipment, and treated by suitable NOx reduction technology, the best of which is SCR if it can be done effectively without storing dangerous ammonia in a system which is as flexible as the power grid is to energy demand.
SCR has been proven to be highly effective at NOx reduction, and SCR units can generally be scaled to the size required for turbines. However, SCR units typically require the use of ammonia as a reducing reagent, and it is a common problem that ammonia is difficult and dangerous to store, especially in populated areas. Thus, the use of ammonia generators such as described in U.S. Pat. No. 7,090,810 to Sun, et al., and U.S. Pat. No. 6,077,491 to Cooper, et al., are often required, but their control for multiple turbine units has not been addressed and can be more costly or difficult than economics may permit in some installations, such as turbines for use with digester and landfill gases.
The digester and landfill gases, which have more traditionally been burned by a flare because of the low quality of the gases, can create costs that are difficult to recover. For these gases, the problem of economics is especially great. Installations requiring more than one turbine cannot presently benefit from a single urea-based ammonia SCR plant. Unfortunately, it has been seen that utilization of a single urea conversion unit for each turbine is the most practical approach.
However, because the demand on the turbines for power fluctuates over time—with daily and seasonal fluctuations—single ammonia generators have not been practical. SCR units typically employ ammonia injection grids (AIGs), which are essentially arrays of distribution pipes with holes arranged through which the ammonia is preferably ejected with a carrier gas to provide sufficient momentum for the gas at each location and thereby achieve uniform distribution of ammonia. When demand is low, immediate decrease of ammonia to one SCR unit for one turbine, will create a temporary excess of ammonia for the others. Without employing storage tanks, which is to avoided for ammonia safety reasons, there is currently no good way to accommodate the fluctuations. Distribution will be adversely affected or excess ammonia will be supplied and result in ammonia slip.
There is a present need for a process, apparatus and system for efficient utilization of urea for selective catalytic reduction (SCR) of NOx, and more particularly for feeding a gasified product of the urea to multiple turbine power units from a single urea gasification unit.
There is a particular need for such a system which converts urea to ammonia, yet maintains the ability to fully control separate SCR units without excessive reagent usage or loss of pollution control effectiveness.